Publication Summary

Geologic conditions around Lake Sawyer were assessed by reviewing existing reports, studying available geologic logs of regional water wells, and logging four borings in which monitoring wells were installed. Based on an initial review of this information, a monitoring network was developed to provide ground water flow and quality information. The network included four monitoring wells, 15 wellpoints, ten domestic wells, and five staff gages.

(Prepared for the Washington State Department of Ecology by Hart Crowser, Seattle, Washington.)

Geologic conditions around Lake Sawyer were assessed by reviewing existing reports, studying available geologic logs of regional water wells, and logging four borings in which monitoring wells were installed. Based on an initial review of this information, a monitoring network was developed to provide ground water flow and quality information. The network included four monitoring wells, 15 wellpoints, ten domestic wells, and five staff gages.

From August 1989 to May 1990, monthly water level measurements were obtained from the network, and approximately quarterly water quality samples were collected from the monitoring wells and wellpoints.

The inputs of total nitrogen (TN) and total phosphorus (TP) to Lake Sawyer via the regional ground water system were assessed by multiplying the average estimated ground water inflow rate (0.1 cfs) by measured background concentrations of these nutrients within the basin. Based on these data and assumptions, the estimated nutrient inputs to the lake from regional ground water system average approximately 50 kgTN/year and 0.8 kgTP/year.

Septic system releases of nitrogen and phosphorus to Lake Sawyer were assessed by monitoring ground water quality within approximately 100 feet of a variety of nearshore septic systems within the basin. Based on a mass balance relationship, removals of TP and TN occurring during transport from the septic tank to the ground water monitoring locations averaged 94 ± 2 percent and 79 ± 11 percent, respectively. Applying these observed removals to the total number of dwellings within the "effective" Lake Sawyer catchment area results in an estimated existing (1989-1990) wastewater loading of approximately 190 ± 90 kgTN/year and 9 ± 4 kgTP/year.